Motor vehicles are being increasingly equipped with driver assistance systems that cover the surroundings by means of sensor systems and derive automatic vehicle reactions from the traffic situation detected in this manner and/or instruct (in particular warn) the driver, wherein a distinction between comfort functions and safety functions is made.
FSRA (Full Speed Range Adaptive Cruise Control) is the most important comfort function as far as present development is concerned. The vehicle adjusts the ego-velocity to the desired velocity predetermined by the driver if said adjustment is possible in the present traffic situation. Otherwise, the ego-velocity is automatically adjusted to the traffic situation.
Besides increased comfort, safety functions will be more and more important in future, wherein a reduction of the length of the brake path in emergency situations will probably play the most important role. The corresponding driver assistance functions range from prefilling the brake automatically for reducing brake latency via an improved Brake Assist System (BAS+) to autonomous emergency braking.
Nowadays, radar sensors are used in most driver assistance systems of the type described above. Said radar sensors reliably operate even in bad weather and are capable of measuring the distance between objects as well as of directly measuring the relative velocity of the objects by means of the Doppler effect.
These radar sensors are still rather expensive, and the detection quality thereof is not perfect, which is very critical particularly with respect to safety functions. Reasons thereof are, for example:                For precisely determining the lateral position of objects, radiation lobes directed in many different directions are required. This is being increasingly realized by using several transmitting and/or receiving antennas integrated in one sensor, wherein a planar design is preferred. Said transmitting and/or receiving antennas are operated in a parallel or quasi-parallel manner. For preventing the antennas from influencing each other, they must be decoupled or isolated very well, which has not been realized in a satisfying manner so far, not even by using expensive circuitry.                    One example are 24-GHz UWB sensors (UWB=Ultra Wide Band). The very restrictive frequency allocation only allows the radiation of a very low transmitting power, which results in the fact that the amount of power unintentionally radiated by the actual receiving antennas on account of insufficient isolation comes close to the amount of power radiated by the transmitting antennas. This may result in problems with respect to the azimuth angle estimation of objects and in sensitivity losses in particular angle regions.                        If several antennas are used, several parallel receiving paths are implemented, which makes analog and digital signal processing very expensive.        Interference coupling or interference radiation (caused by other systems) in the radar frequency range or in the range in which the low-frequency part of the electronic evaluation unit operates may result in incorrect detection and thus in wrong reactions.        